1. Field of the Invention
The present invention relates to high-voltage capacitors and, more particularly, to a capacitor having improved surface breakdown voltage performance using novel method for applying laser or conductive ink marking which increases capacitor surface breakdown voltage.
2. Description of the Prior Art
Capacitors are second only to resistors as electronic circuit elements and are used in virtually every type of electronic circuit. In a typical application, capacitors are used as coupling capacitors in electronic circuits to block a direct current (DC) voltage while allowing alternating current (AC) currents to pass. They are also used as bypass capacitors to prevent the voltage at a circuit node from changing. These two applications do not depend on the exact value of the capacitance. In other applications, capacitors are also used in inductor-capacitor (LC) resonant circuits, in resistor-capacitor (RC) bridges, and as timing elements. In these applications the precise value of the capacitor is usually significant. Capacitors may also be used for accurate charge storage; for example, in sample-and-hold circuits or frequency-to-voltage converters. In these applications, the quality of the dielectric is more important than the capacitance, as long as the latter is stable.
Capacitors are used store charge for discharge through lamps, as in photographic flash lamps or stroboscopes. Capacitors are also used in filtering applications, both in power-supply filters and signal processing applications, often in conjunction with operational amplifiers or other analog circuit components. In power circuits, capacitors may be used to shift phase, as in capacitor-start motors, or for power factor compensation in the case of inductive loads. Finally, capacitors are extremely important in integrated circuits, where they are structured as metal or polysilicon films with silicon dioxide dielectrics on silicon.
A capacitor is typically constructed from two or more conducting electrodes, each of area A and separated by distances d. In most cases, the separations between the conducting electrodes are maintained by sheets of dielectric material of thickness d and having a dielectric constant k. The capacitance of this parallel-plate capacitor (in Farads) is given by C=ekAn/d F, where A is in m2 d is in meters, n is the number of parallel conducting electrodes, and pi=3.1415926. In SI units the constant is e=4(pi)/c2=8.854×10−12 F/m. In units of centimeters, the capacitance is given by C=kAn/4(pi)d, where A is now in cm2 and d in cm.
While the net charge on a capacitor under an applied voltage is zero, equal and opposite charges ±Q are induced on adjacent pairs of electrodes. The charge on the capacitor is thus taken to be |Q|, the absolute value of the charge. If one terminal of a capacitor has a polarity marking, say a +, then Q>0 means there is a positive charge Q on the corresponding electrode; similarly, Q<0 means there is a negative charge on that electrode. Further, given a current i=dQ/dt, a positive current flowing into the positively marked terminal will cause the voltage V across the capacitor to increase. The voltage V is the potential difference between the electrodes, and this voltage is positive if the potential of the positively marked plate is higher than the potential of the other.
In an ideal capacitor, the voltage V is proportional to the charge Q, and the constant of proportionality is defined as the capacitance C. More particularly, Q=CV, or C=Q/V. Most capacitors are close to ideal if the voltage is not excessive and does not vary too rapidly. Conventionally, the units are defined so that a unit charge is one coulomb, and unit voltage is one volt, or one joule/coulomb. The corresponding unit of capacitance is therefore one Farad, or one coulomb per volt. The Farad is an impractically large unit, so the microfarad (10−6 Farad) or picofarad (10−12 Farad) is commonly used in practice.
As discussed above, most practical capacitors include a dielectric material between the electrodes. The electric field E=V/d between the electrodes passes through the dielectric, terminating on the induced charges charges on the electrodes. Within the dielectric, the charges are elastically bound to their equilibrium positions and can only move slightly in response to the electric field. The positive charges move in the direction of the electric field, and the negative charges move in the opposite direction.
When placed between the parallel electrodes of a capacitor, the dielectric is polarized by the electric field. In order to maintain the electric field at V/d, the extra charge at the surfaces of the dielectric is balanced by equal and opposite charges on the electrodes. At the positive electrode, which had a charge density of E/4(pi) without the dielectric, a charge density P is added resulting in a total charge density of E/4(pi) +P=(E+4(pi)P)/4(pi). The combination E+4(pi)P is the total charge per unit area on the electrode and is analogous to E, the charge per unit area in the absence of the dielectric.
The combination D=E+4(pi)P is called the electric displacement. The flux of D through a closed surface is 4(pi) times the amount of free charge enclosed. The free charge is the total charge less the charge contributed by polarization, and represents that part of the charge that can move, while the polarization charge remains fixed.
A dielectric constant k can be defined in terms of the electric displacement as D=kE. The charge on the capacitor electrodes in the presence of the dielectric is thus s=D/4(pi)=kE/4(pi). Therefore, the total charge Q is increased by the factor k and the capacitance is now C=kAn/4(pi)d. Likewise, introducing a uniform dielectric reduces the electrode potentials by a factor 1/k, since the electric fields must be reduced by this factor to keep the same charge Q. Therefore, the effect of adding the dielectric is to increase the capacitance C while keeping area A and separation d constant.
The principal dielectric materials currently in use include mica, Mylar, polystyrene, polypropylene, polycarbonate, polyester, ceramic, aluminum electrolytic, tantalum electrolytic, and gold double layer.
The maximum voltage at which a capacitor can be used is determined by the formation of an electrical discharge in the dielectric. The heat produced by the discharge usually damages the capacitor, except in the case of an air or liquid dielectric, in which little permanent damage may be done. The dielectric strength is the maximum voltage difference that a given thickness of dielectric can sustain without electrical breakdown. Capacitors having improved voltage breakdown performance are in continuous demand in modern electronic circuits and devices, including flat panel and plasma televisions, telecommunications equipment, and military electronics systems.
High-voltage capacitors rated at 2,000 volts or more typically use a floating electrode design to increase effective dielectric thickness and to control electric field gradients within the capacitor. A key parameter of a high-voltage capacitor is the surface breakdown voltage, which defines the maximum voltage at which a capacitor can be used. The surface breakdown voltage is caused by inter-terminal current leakage across the outer surface of the capacitor body, as opposed to discharge between the electrodes and through the dielectric. Thus, the delectric constant of air (k=1.0006) is a key determinant of the surface breakdown voltage.
Most capacitors are marked with the capacitor value and voltage limit. For example, one common scheme uses three digits to indicate a value in picofarad. The first two digits are significant figures, and the final digit is a decimal exponent. Thus, 104 translates to 10 followed by 4 zeros, 100000 picofarads 0.1 microfarads; 472 means 4700 picofarads or 0.0047 microfarads. Other letters and numbers on disc ceramic capacitors are usually the temperature range or value tolerance. For example, 0.1Z Y5S means 0.1 microfarads, tolerance −20%+80%, useful temperature range −30° C. to 85° C., ±22% variation in value over this range. At one time, small capacitors were labeled with colored stripes like the resistor code, and mica capacitors in molded packages had six or three colored dots interpreted the same way; however, there is no uniform series of preferred values for capacitors as there is for resistors.
In addition to capacitor value, modern certified safety capacitors use a laser or conductive ink mark specific to each manufacturer of those safety capacitors to signify that they are safety certified by either the TUV or UL. A typical Multi-Layer Chip Capacitor (MLCC) marking is located on the top center of the device package. This location reduces the creepage, or arc distance, between the device terminals by effectively dividing the inter-terminal distance in half. This occurs because many of the marking inks use metal or carbon particles for color and are therefore slightly electrically conductive. In addition, the shape and size of the mark, along with its orientation, can also have an impact on surface breakdown voltage. Thus, conventional laser marking processes effectively create a slightly conductive island on the dielectric surface in the middle of the device between the terminals, thereby reducing the surface breakdown voltage.
Accordingly, there is a need capacitors having improved surface breakdown voltage performance, and a method for applying laser or conductive ink marking to capacitors which does not degrade or reduce capacitor surface breakdown voltage, is easy to apply using existing laser marking technologies and apparatus, and which results in a mark that is legible and clear.